1. Technical Field
The present disclosure relates to a photolithography method, and more specifically to a photolithography method using a chemically-amplified resist.
2. Discussion of Related Art
In certain integrated circuit manufacturing technologies corresponding to particularly high integration levels, sensitizing radiations in the deep ultraviolet range at a wavelength smaller than or equal to 250 nm have to be used during photolithography steps. For example, KrF excimer lasers are used for 248-nm wavelengths and ArF excimer lasers are used for 193-nm wavelengths. Current resists, for example resists of novolak/naphtoquinone diazide type, cannot be used for such wavelengths since they absorb the radiation and the resist is not sensitized across its entire thickness. A solution is to use chemically-amplified resists.
Chemically-amplified resists are materials capable of generating an acid compound in regions exposed to a radiation, for example, an ultraviolet radiation, and to change solubility in a development solution, by reaction with the acid compound, in exposed regions with respect to non-exposed regions.
Chemically-amplified resists comprise polymer chains intrinsically soluble in a determined development solution, generally an alkaline development solution. Resists soluble in an alkaline development solution for example comprise polymer chains supporting —OH or —COOH functions. The polymer chains are made insoluble in the development solution by dissolution inhibitors incorporated in the resist. Such dissolution inhibitors for example are groups bonded to the polymer chains by acid functions —OH and —COOH of these chains. The dissolution-inhibiting groups for example are hydrophobic groups in the case where the development solution is an aqueous solution. Chemically-amplified resists further comprise precursors of compounds capable of deactivating dissolution inhibitors. Such resists are called chemically-amplified resists since a same precursor produces a compound capable of deactivating several dissolution inhibitors.
In the case where a KrF excimer laser is used as sensitizing radiation source, resists having as a backbone a polyhydroxy styrene functionalized by lateral groups are generally used. In the case where a radiation source with a sensitization of lower wavelength is used, for example, an ArF excimer laser, resists having an alicyclic, monocyclic, or polycyclic hydrocarbon structure are preferably used, compounds comprising aromatic groups and many unsaturated bonds having too high a radiation absorption to enable the creation of high-resolution patterns.
The precursors of compounds capable of deactivating dissolution inhibitors for example are acid compound precursors.
Such chemically-amplified resists are for example formed of 95% of polymer chains and 5% of acid compound precursors. Acid compound precursors for example are onium salts, for example triphenylsulfonium salts or iodonium salts. Sulfonates may also be used, for example, imidosulfonates or oximesulfonates, diazodisulfones, disulfones, or o-nitrobenzyl sulfonates.
FIGS. 1A-1C and 2A-2B illustrate successive steps of a conventional photolithography method using a chemically-amplified resist of the above-described type.
FIG. 1A is a cross-section view illustrating a step of deposition, on the upper surface of a wafer 1, of a chemically-amplified resist layer 3. The resist deposition step may be preceded by a step of deposition of a coupling agent, for example, hexamethyldisilazane (HMDS), intended to improve the bonding of resist 3 to the upper surface of wafer 1. The resist deposition is then performed by centrifugation of the resist dissolved in a solvent. Once deposited, the resist is dried and submitted to a heating enabling to increase its density and to relax the stress present therein.
After the resist has been deposited, as illustrated in FIG. 1B, the resist is exposed to a sensitizing radiation. The resist exposure is performed through a mask 5 enabling to define exposed regions 7 and non-exposed regions 9 of the resist. In exposed regions 7 of the resist, the sensitizing radiation causes the generation of acid compounds by the acid compound precursors.
After the resist exposure, at the step illustrated in FIG. 1C, the resist is heated. The heating of the resist causes a reaction between the acid compounds, generated during the exposure step, and the polymer chain dissolution-inhibiting groups. The resist heating temperature to cause the reaction between acid compounds and dissolution-inhibiting groups depends on the group activation energy. This temperature is generally greater than approximately 90° C., and preferably ranges between 100 and 140° C. An acid compound for example reacts with a few hundreds of dissolution-inhibiting groups. When attacked by acid compounds, the dissolution-inhibiting groups separate into polymer chains. The reaction causes a degassing and a rearrangement of the polymer chains, which results in a decrease in the thickness of exposed regions 7 of the resist, as shown in FIG. 1C. During this resist heating step, the polymer chains, partially or totally freed of their dissolution-inhibiting groups, become soluble again into the alkaline development solution.
During the step of heating after exposure, acid compounds diffuse towards the non-exposed resist regions. To limit this parasitic diffusion phenomenon, a solution is to use chemically-amplified resists having dissolution inhibiting groups of low activation energy. The resist heating temperature can thus be decreased, for example, to a value approximately ranging from 80 to 90° C.
It should be noted that the acid compounds photogenerated in the resist regions exposed to the sensitizing radiation must absolutely not be neutralized before having been able to play their dissolution inhibitor deactivation role. All possible precautions are thus taken to avoid for the exposed and non-developed resist to be contaminated by alkaline compounds, amines, or organic compounds. The steps illustrated in FIGS. 1A to 1C are thus carried out under an atmosphere protected from such contaminants. In particular, these steps are carried out in photolithography equipment capable of filtering compounds such as alkaline compounds, amines, or organic compounds. The content of such compounds in the atmosphere is further strictly controlled in the clean room area where the photolithography equipment is installed, and where wafers covered with exposed and non developed resist may be momentarily stored.
A next step comprises developing the resist. This may performed according to one or the other of the variations illustrated in FIGS. 2A and 2B.
In the case of FIG. 2A, the resist has been developed in an aqueous alkaline tetramethyl ammonium hydroxide (TMAH) solution, for example, at a 2.38% concentration. The polymer chains of the exposed regions have recovered, after the resist heating, their solubility in the alkaline development solution. After development, only non-exposed resist regions 9 remain on the upper surface of the wafer, exposed regions 7 having been removed.
In the case of FIG. 2B, the resist has been developed in an organic solvent, for example, a solvent comprising methyl-n-amyl ketone or butyl acetate. In this case, the lack of solubility of the polymer chains having recovered their —OH and —COOH functions in an organic solvent is used. The polymer chains of the non-exposed regions are not soluble in an aqueous solution, since they substantially support as many dissolution inhibiting groups as originally, but are soluble in an organic solution. The exposed regions are not soluble in an organic solution. As illustrated in FIG. 2B, after the step of development of the resist by an organic solvent, only exposed regions 7 of the resist then remain on the upper surface of the wafer.
U.S. Patent Application Publication No. 2004007382 describes a photolithography method of the above-described type using a chemically-amplified resist and comprising two additional steps after the step of resist development by an aqueous solution. After development, the resist regions which were protected by the mask on exposure to the sensitizing radiation remain at the surface. The first additional step comprises exposing the resist to a second sensitizing radiation, without using any mask. The second additional step comprises neutralizing the acid compounds generated in the resist at the second exposure thereof. These two additional steps enable to avoid the flowing of the resist after development, in etching and ion implantation steps during which the wafer is brought to a temperature capable of reactivating the diffusion of residual photogenerated acid compounds and their reaction with dissolution-inhibiting groups, which makes the resist less resistant. Such a resist flow may result in altering the size of the etched or implanted patterns.
The use of chemically-amplified resists supporting dissolution-inhibiting groups of low activation power, to limit the parasitic diffusion of acid compounds during the heating step however raises a new issue. Indeed, in such resists, acid compounds are capable of reacting with dissolution-inhibiting groups in the absence of any heating, that is, at ambient temperature, even if this decreases the reaction kinetics. Now, a non-negligible time that cannot be shortened always elapses between the end of the heating step and the beginning of the development step. Such delays between steps within the photolithography equipment may result in a parasitic diffusion of acid compounds. Further, for practical reasons, associated with the organization of an integrated circuit manufacturing plant, several hours, or even several days, may elapse between the end of the step of heating after exposure and the beginning of the development step. Acid compounds then risk continuing to react in uncontrolled fashion with dissolution-inhibiting groups, even in the absence of any heating. As a result, the size of the exposed regions of the resist layer risks becoming too small with respect to the expected size. Another consequence is that acid compounds diffuse towards the non-exposed regions of the resist, and react in these regions with dissolution-inhibiting groups. The longer the waiting time before development, the less the dimensions of the resist patterns remaining on the upper surface of the wafer will correspond to the dimensions of the patterns of the initial mask. If the differences between dimensions of the resist patterns and the dimensions of the mask patterns are too large, for example, if these differences correspond to more than 10% of the dimensions of the smallest patterns of the mask, all the steps of the photolithography method will have to be resumed.
There thus is a need for a photolithography method using chemically-amplified resists providing resist pattern dimensions after development which substantially corresponds to the dimensions defined on the mask used during the resist exposure, or dimensions taking into account a given intentional dimensional shift between the dimensions defined on the mask and those reproduced in the resist, whatever the delay or the waiting delay between the end of the heating step after exposure and the beginning of the development step.